This invention relates to the field of pneumatic actuators and more particularly to automatic closed loop control of the position of a pneumatic actuator, Generally speaking, closed loop control of actuator positioning is a highly developed art. However, the prior art is concerned primarily with feedback control of linear systems. System non-linearities inherently associated with the compressibility of the working gas render the teachings of such prior art largely non-applicable to the automatic control of a pneumatic actuator.
The published work in the field of closed loop control of pneumatic actuators has nearly always attempted to achieve tight, stable positioning through the use of linearizing approximations. Typical assumptions have included: small actuator movement from a center position, chamber volumes which are constant and equal, small pressure differentials, and chamber pressures which are constant and equal. Such approximations are appropriate for certain specialized applications, but satisfactory control cannot be achieved when those and other linearizing approximations are applied to controllers for low pressure, long excursion actuators. In particular, linearizing assumptions have been found unsuitable for closed loop position control of pneumatic actuators employed in large industrial applications and having a piston travel distance in the order of about 200 in. and differential chamber pressures ranging from 0 to 100 psig. Thus while various closed loop schemes have been proposed, open loop positioning of such actuators has been the norm. Moreover, prior art controllers have generally generated trajectory commands for pneumatic actuators on a basis independent of the achieved responsive motion of the actuator. This has produced unsatisfactory performance characterized by dynamic stiffness, large positioning errors and performance variations along the stroke length.